1. Field of the Invention
The present invention relates to an analyser for detecting explosives. It is particularly, but not exclusively, concerned with the detection of explosives in luggage in e.g. an airport security system.
2. Summary of the Prior Art
It is known that explosives generally contain a higher amount of nitrogen than normal objects to be found in luggage, and therefore consideration has been given to methods for analysing the amount of nitrogen present in luggage to detect the presence of explosive. When nitrogen is subject to neutrons, it emits gamma-rays at predetermined frequencies, by detection of such gamma-rays, it is possible to determine the nitrogen concentration of a particular sample. Thus, if it is found that there is an excessive level of nitrogen in any particular part of an item of luggage, that item becomes suspect, and therefore should be subject to further investigation.
One proposal for investigating explosives by mean of neutron analysis is disclosed in GB 1392169. In this document, an item of luggage is positioned with an enclosure, the walls of which contain gamma-ray detectors. At one point in the walls, there is a neutron source which, subjects the item of luggage within the enclosure to neutrons, and the gamma-rays generated are then detected by the gamma-ray detectors. Since the source and the detectors which detect gamma rays due to the neutrons from that source are on the same side of the item of luggage, this configuration is known as scatter geometry. In GB 1392169 the gamma-ray detectors are axially elongate, so that their maximum detection efficiency is axial, so they may be considered as detecting gamma-rays from only a limited area of the item of luggage.
In our UK patent number 2217009, we described a system for detecting explosives in which each item of luggage is passed adjacent a plurality of detectors, arranged approximate one or more neutrons sources. The output of the detectors were then analysed over a series of time intervals, so that each time interval corresponded to the movement of only part of the luggage that passed the detectors. This was found to give good detection efficiency.
One problem with any system which uses the presence of nitrogen in explosive for detection is that the standard items in luggage, such as clothing, may have nitrogen contents which are not significantly less than the nitrogen content of the explosive. Some types of explosive have high nitrogen contents, and these are relatively easy to detect. Other types of explosive, however, may have a lower nitrogen content (e.g. 18%) and it is difficult to detect the difference between this and e.g. wool which has a nitrogen content of around 13%.